Firefox only (Windows/Mac/Linux): Firefox is a powerful browser with extensions to do almost anything, but it's never been accused of being light on resources—and limiting your history size can help.

Every time your browser requests a page or image from anywhere, that data is stored in your browser history for a default of 90 days—for most of us that translates to an enormous amount of data stored in the internal history database. The Mac Tips and Tricks weblog has a quick and very useful tip to make Firefox load more quickly—just visit Preferences -> Privacy -> History and turn the dial down to a more reasonable 5-10 days. You'll need to clear your history or restart Firefox after you make this change, but in my testing the startup speed improved significantly.